1. Field of the Invention
The present invention relates to a flow sensor that not only detects a flow rate of a fluid but also obtains a detection signal that varies depending on a flow direction of the fluid by electrically detecting an amount of heat transmitted to the fluid from a heating element disposed in the fluid, and to a control system of an internal combustion engine.
2. Description of the Related Art
Generally, in a thermal flow sensor, a heating element and a fluid temperature detection element are disposed in a fluid and a bridge circuit is formed of the heating element, the fluid temperature detection element, and multiple resistors, so that a heating current supplied to the heating element is controlled for the bridge circuit to constantly maintain an equilibrium condition.
Accordingly, a temperature of the heating element is constantly maintained at a temperature predetermined degrees higher than a temperature of the fluid detected by the fluid temperature detection element.
A type used extensively is so-called a heating current detection type that uses a heating current as a detection signal by exploiting dependency of a heating current on a flow rate of the fluid. However, this detection signal is to detect an absolute value of the flow rate and there is a problem that both of a forward flow and a backward flow have the same value independently of a flow direction of the fluid.
In order to solve this problem, there is a method of obtaining a detection signal that varies depending on a flow direction of the fluid by forming bridge circuits using two sets of heating elements and fluid temperature detection elements disposed upstream and downstream in a flow direction of the fluid so as to undergo mutual thermal interference and using a difference between heating currents supplied to the respective heating elements as a detection signal (for example, see Patent Document 1).
[Patent Document 1] JP-A-11-201793
The method of Patent Document 1, however, requires two sets of bridge circuits and therefore requires a complex and large-scale circuit, and such a requirement becomes an obstacle to a reduction in size and cost.
Further, in order to detect a flow rate with high accuracy, the two bridge circuits need to have characteristics that are quite alike. It is therefore extremely difficult to manufacture these bridge circuits.